1. Field of the Invention
The present invention relates to a ceramic and metal joining structure, as a structure for joining a metal shaft to a ceramic turbine wheel of a turbocharger by the use of a metal sleeve.
2. Description of the Prior Art
Ceramic materials have a very high hot hardness but are deficient in strength such as fracture toughness. For this reason, in case, for example, where a turbine wheel of a turbocharger for an internal combustion engine is made of ceramic, it needs to be joined with a metal shaft. An example of this kind of ceramic and metal joining structure is shown in FIG. 9. In the joining structure, a ceramic shaft 101 integral with a ceramic turbine wheel is joined at its end with a corresponding end of a metal shaft 102 by interposing therebetween a buffer member 103, while being received within a metal sleeve 104 together with the metal shaft 102 at their matched ends. The metal sleeve 104 is attached to the ceramic shaft 101 and the metal shaft 102 by brazing or shrink fit.
A disadvantage of the foregoing joining structure is that the ceramic shaft 101 is liable to fracture in the place adjacent a joint end 101a and therefore poor in durability as well as unreliable in operation. This is because high stress concentration is liable to occur in the ceramic shaft 101 in the place adjacent the joint end 101a due to the fact that stresses are produced in the ceramic shaft 101 by a compressive force applied thereto from the brazing metal or the shrink-fitted metal sleeve 104 and to the fact that thermal stresses are liable to be produced in the ceramic shaft 101 in the place adjacent the joint end 101a since the joint end 101a is exposed to high temperature gases when in use.